The original Garrysmod background with 'Where's Your Head At' by the 'Basement Jaxx' for background music.Extremely fitting, extremely good music.This includes the original background encase you have installed another one and no longer have it.Enjoy. -Zarradeth

This download looks like an unpacked addon. You can either convert the files to an addon or merge the folders in the zip with the folders in your Garry's Mod folder.

Convert to Addon (Recommended)

You should end up with a directory structure similar to below:

steamapps

common

garrysmod

garrysmod

addons

wheresyourheadat_background.zi <-- Make this folder!

e.g.

lua

maps

materials

models

etc

Or, Merge Folders (Not recommended)

This option makes it difficult to remove the content should you no longer want it. Merge any folders from the zip with the equivalent folders in your Garry's Mod folder.